Electrospinning for the formation of fine fibers has been actively explored recently for applications such as high performance filters [1,2] and biomaterial scaffolds for cell growth, vascular grafts, wound dressings or tissue engineering [2-4]. Fibers with nanoscale diameter provide benefits due to their high surface area. In this electrostatic technique, a strong electric field is generated between a polymer solution contained in a glass syringe with a capillary tip and a metallic collection screen. When the voltage reaches a critical value, the charge overcomes the surface tension of the deformed drop of suspended polymer solution formed on the tip of the syringe, and a jet is produced. The electrically charged jet undergoes a series of electrically induced bending instabilities during passage to the collection screen that results in stretching [5-7]. This stretching process is accompanied by the rapid evaporation of the solvent and results in a reduction in the diameter of the jet [8-12]. The dry fibers accumulated on the surface of the collection screen form a non-woven mesh of nanometer to micrometer diameter fibers even when operating with aqueous solutions at ambient temperature and pressure. The electrospinning process can be adjusted to control fiber diameter by varying the charge density and polymer solution concentration, while the duration of electrospinning controls the thickness of the deposited mesh [8-13].
Protein fiber spinning in nature, such as for silkworm and spider silks, is based on the formation of concentrated solutions of metastable lyotropic phases that are then forced through small spinnerets into air [14]. The fiber diameters produced in these natural spinning processes range from tens of microns in the case of silkworm silk to microns to submicron in the case of spider silks [14]. The production of fibers from protein solutions has typically relied upon the use of wet or dry spinning processes [15, 16]. Electrospinning offers an alternative approach to protein fiber formation that can potentially generate very fine fibers. This would be a useful feature based on the potential role of these types of fibers in some applications such as biomaterials and tissue engineering [17]. Electrospinning has been utilized to generate nanometer diameter fibers from recombinant elastin protein [17] and silk-like protein [18-20]. Zarkoob et al. [21] have also reported that silkworm silk from Bombyx mori cocoons and spider dragline silk from Nephila clavipes silk can be electrospun into nanometer diameter fibers if first solubilized in the organic solvent hexafluoro-2-propanol (REP).
Silk is a well described natural fiber produced by the silkworm, Bombyx mori, which has been used traditionally in the form of threads in textiles for thousands of years. This silk contains a fibrous protein termed fibroin (both heavy and light chains) that form the thread core, and glue-like proteins termed sericin that surround the fibroin fibers to cement them together. The fibroin is a highly insoluble protein containing up to 90% of the amino acids glycine, alanine and serine leading to β-pleated sheet formation in the fibers [22].
The unique mechanical properties of reprocessed silk such as fibroin and its biocompatibility make the silk fibers especially attractive for use in biotechnological materials and medical applications [14, 23].
Electrospinning silk fibers for biomedical applications is a complicated process, especially due to problems encountered with conformational transitions of silkworm fibroin during solubilization and reprocessing from aqueous solution to generate new fibers and films. The problem with conformation transition is due to the formation of β-sheets which result in embrittled materials. Additionally, organic solvents typically used in silk electrospinning, as well as foam, film or mesh formation, pose biocompatibility problems when the processed materials are exposed to cells in vitro or in vivo.
Silk blends have been extensively studied with respect to film formation. Blends with polyacrylamide [26], sodium alginate [27], cellulose [28,35], chitosan [29,36,37], poly(vinyl alcohol) [30,38,39], acrylic polymers [31], poly(ethylene glycol) (300 g/mol [40] or 8,000 g/mol [41]) poly(ε-caprolactone-co-D,L-lactide) [42], and S-carboxymethyl keratin [43] have been studied to improve the mechanical or thermal stability or membrane properties of silk films.
Unfortunately, none of these blends have proven successful in overcoming problems associated with processing or reprocessing silk protein, e.g., embrittlement, and, therefore, new methods, especially organic solvent free methods, are needed.